Layer by layer self-assembly of large response molecular electro-optic materials by a desilylation strategy

ABSTRACT

The preparation of robust, thin film materials with large second-order optical nonlinearities through the covalent self-assembly of chromophoric compositions and innovative use of silyl chemistry.

This application claims the benefit of prior U.S. provisionalapplication No. 60/191,360, filed Mar. 22, 2000, the entirety of whichis incorporated herein by reference.

The United States government has rights in this invention pursuant toGrant No. DMR 9632472 from the National Science Foundation and Grant No.ONR N00014-95-1-1219 from the Office of Naval Research, both toNorthwestern University.

BACKGROUND OF THE INVENTION

Organic molecule-based optical technology components such aselectro-optic modulators promise greatly increased rates of informationtransmission by enhancing network speed, capacity, and bandwidth fordata networking and telecommunications. There is a vast need forincreased data handling density in photonic devices, and futurehigh-speed fiber-optic networks will be required to carry orders ofmagnitude more data than possible with conventional electronic systemsand to easily handle phone calls, e-mail, webpages, video,high-definition television (HDTV) signals. Therefore, the development ofnew electro-optic/second-order nonlinear optical (NLO) materials withexcellent optical, thermal, and chemical properties is a topic of greatcurrent scientific interest. Of the synthetic approaches investigated,Langmuir-Blodgett (LB) film transfer, polymer poling, and self-assembly(SA) have been used to obtain soft thin films with a variety ofelectro-optic response properties.

Intrinsically acentric SA organic materials can exhibit far higherelectro-optic coefficients and lower dielectric constants thanestablished inorganic materials (e.g., lithium niobate and potassiumdihydrogen phosphate), and do not require electric field poling. (Forreviews see the Chem. Rev. special issue on Optical Nonlinearities inChemistry, ed: D. M. Burland, 1994, 94, 1-278.) Chemisorptive siloxaneSA was originally developed by Sagiv (R. Moaz, J. Sagiv, Langmuir 1987,3, 1034-1044) and is known to yield robust, densely packed organic filmson hydroxylated surfaces. Self-assembled mono- and multilayeredstructures are accessible with relatively simple hydro- or fluorocarbonchains (A. Ulman, An Introduction to Ultrathin Organic Films: fromLangmuir-Blodgett to Self-Assembly, Academic Press, Inc. San Diego,1991), whereas fabrication of complex superlattices is relatively rare.For instance, Katz, et al. reported the formation of acentricmultilayers by alternately depositing layers of phosphonate-substitutedazo chromophores and zirconyl salts. (H. E. Katz, W. L., Wilson, G.Scheller, J. Am. Chem. Soc., 1994, 116, 6636-6640.)

Previous studies showed that robust, acentric mono- and multilayeredstructures composed of stilbazolium and related chromophores (See, FIG.1A-C) and exhibiting very large NLO response properties (χ⁽²⁾=150-200pm/v) can be obtained by a three-step procedure, the second of whichinvolves a cumbersome spin-coating of chromophore solutions, followed byvacuum oven treatment. The synthetic tools available for the formationof surface bound functional organic multilayered structures are ratherlimited in comparison to the tremendous variety of reactions known insolution phase organic chemistry.

Organic monolayers containing nonpolar end-groups are chemically inerttowards binding of chloro- or alkoxy silanes from the solution phase.Furthermore, regeneration of a new reactive hydroxylated or carboxylatedsurface is an essential requirement for the iterative growth ofsiloxane-based multilayers. In order to form highly ordered multilayeredstructures, a constant and/or a large density of reactive sites must bepresent at the surface of each added layer. To this end, it was recentlydemonstrated that 9-fluorenylmethoxycarbonyl (Fmoc) anddi-p-methoxytrityl (DMT) groups can be used for the reversibleprotection of amine and hydroxyl-terminated alkanethiol monolayers ongold substrates. (Frutos, J. M. Brockman, R. M. Corn, Langmuir 2000, inpress.) Hydroboration or oxidation of terminal double bonds (L. Netzer,J. Sagiv, J. Am. Chem. Soc. 1983, 105, 674-676; R. Moaz, S. Matlis, E.DiMasi, B. M. Ocko, J. Sagiv, Nature 1996, 384, 150), hydrolysis ofphosphonate esters (G. A. Neff, C. J. Page, E. Meintjes, T. Tsuda, W. C.Pilgrim, N. Roberts, W. W. Warren, Jr., Langmuir 1996, 12, 238-242),reduction of methyl esters (S. R. Wasserman, Y. T. Tao, G. M.Whitesides, Langmuir 1989, 5, 1074-1087; M. Pomerantz, A. Segmuller, L.Netzer, L. Sagiv, J. Thin Solid Films 1985, 132, 153-162), andphotolysis of organic thin films (R. J. Collins, I. T. Bae, D. A.Scherson, C. N. Sukenik, Langmuir 1996, 12, 5509-5111) have also beenused to create hydroxyl-terminated surfaces.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide chromophores, non-linear optic materials and/or integrateddevices and methods for the production and/or assembly of suchchromophores, materials and devices, thereby overcoming variousdeficiencies and shortcomings of the prior art, including those outlinedabove. It will be understood by those skilled in the art that one ormore aspects of this invention can meet certain objectives, while one ormore other aspects can meet certain other objectives. Each objective maynot apply equally, in all its respects, to every aspect of thisinvention. As such, the following objects can be viewed in thealternative with respect to any one aspect of this invention.

It is an object of the present invention to provide a new, efficientself-assembly technique to prepare electro-optic materials that can beused for the growth of robust, intrinsically polar arrays of covalentlybound high-hyper-polarizability (β) chromophores directly on silicon orother suitable substrates, so as to allow formation of a variety ofelectro-optic and related integrated devices. Accordingly, it can alsobe an object of the present invention to integrate the chromophores,composition, films and/or materials of this invention into device typestructures such as planar waveguides (for frequency doubling) andelectro-optical modulators.

It can also be an object of the present invention to integrate a wetchemical approach into a straightforward methodology for theself-assembly of large response electro-optic superlattices.

It can also be an object of the present invention to providesecond-order chromophoric thin films, as can be constructed withsub-nanometer level control of layer dimension, with microstructuralacentricity preserved, layer by layer, during self-assembly.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and its descriptions ofvarious preferred embodiments, and will be readily apparent to thoseskilled in the art having knowledge of various non-linear opticmaterials, devices and assembly/production techniques. Such objects,features, benefits and advantages will be apparent from the above astaken into conjunction with the accompanying examples, data, figures andall reasonable inferences to be drawn therefrom, alone or withconsideration of the references incorporated herein.

In part, the present invention is a method of using silyl chemistry tocontrol the reactivity of a self-assembled molecular electro-opticmaterial. The method includes (i) providing an electro-optic materialhaving a silyl-derivatized chromophore; (ii) desilylating thechromophore to generate terminal hydroxy functionalities, and (iii)reacting the hydroxy functionalities with a reagent having at least onesilicon moiety. In preferred embodiments, the chromophore is a high-βchromophore and/or is derivatized with a trialkylsilyl protecting group.Such protecting groups useful with the present invention are limitedonly by the availability of the corresponding silane precursor compoundand/or the effectiveness of the resulting protecting group in theself-assembly procedures described herein. Effectiveness is, in part,based on the deprotection of the hydroxy functionality and removal ofthe protecting group. Various deprotecting agents are available andwould be well known to those skilled in the art of silyl chemistry andits integration into the present invention. Preferably, anionic fluoridereagents can be used with good effect, in particular quaternary ammoniumfluoride reagents.

In part, the present invention is also a method of using silyl chemistryto generate a hydrophilic surface for molecular self-assembly of anelectro-optic material. Such a method includes (i) providing anelectro-optic material comprising a high-β chromophore film withterminal trialkylsiloxy moieties, (ii) desilylating the film to generateterminal hydroxy functional groups, and (iii) reacting the terminalhydroxy functional groups with a siloxane capping agent. With referenceto the discussion, above, the trialkylsiloxy moieties correspond to theaforementioned silyl protecting groups and are derived from theappropriate silane reagents upon reaction with the chromophore material.Desilylation can be achieved as discussed elsewhere, with the resultantterminal hydroxy groups reactive with a reagent having at least onesilicon moiety. In preferred embodiments, such a reagent is a siloxane.Octachlorotrisiloxane is an especially preferred siloxane capping agent,but other molecular components can be used with similar effect. Suchcomponents include, without limitation, the bifunctional siliconcompounds described in U.S. Pat. No. 5,156,918, at colum 7 and elsewheretherein, incorporated by reference herein in its entirety. Other usefulcomponents, in accordance with this invention include thosetrifunctional compounds which cross-link upon curing. Reaction betweenthe terminal hydroxy groups and the capping agent provides a siloxanebond sequence between the chromophore film/layer and capping layer.

Deprotection of a chromophore followed by coupling with a capping agentprovides a siloxane bonded bilayer. Sequential repetition of thissynthetic sequence can be used as a method for assembling amulti-layered non-linear optical material. With an initial chromophorelayer coupled to a suitable substrate, the resulting plurality ofbilayers can be incorporated into a waveguide device. Such devices andrelated integrated device structures can be prepared as described hereinor as, otherwise provided in U.S. Pat. No. 6,033,774 (and in particularcolumns 15-16 and FIGS. 6 a-6 d thereof), incorporated by referenceherein in its entirety.

In part, the present invention is also a chromophore composition and/ormaterial with non-linear optical properties. Such a composition has thestructural formula (Ch)XR_(n), wherein (Ch)X is a chromophoresubstructure and X is a heteroatom, R is a trialkylsiloxyalkyl moietyand _(n) is the number of moieties meeting the valence requirement ofthe corresponding heteroatom. Preferably, the heteroatom is oxygen ornitrogen, but can be any electron-rich heteroatom. Likewise, inpreferred embodiments, the chromophore compositions of this inventioninclude and can be represented by any of the structural formulasprovided herein. See, in particular, the formulas of FIGS. 1, 2, 11 and15. With respect to the latter, such substructures can be modified asdescribed herein to provide the inventive chromophore structures.Furthermore, the silyl chemistry of this invention can be applied toother chromophore systems (e.g. FIG. 1) known in the art, such systemssynthetically modified as necessary by well-known procedures to takeadvantage of the self-assembly strategies described herein. For example,various other systems known in the art are described in U.S. Pat. Nos.5,156,918, 5,834,100 and 6,033,774, each of which is incorporated hereinby reference in its entirety, but especially with regard to eachrespective discussion of the corresponding chromophore and/or conductivemolecular components.

In summary, the present invention is directed a new deposition approachfor assembly of covalently bound thin organic films having excellentelectro-optic response properties. In particular, the solution-phaseprotection-deprotection of hydroxyl groups as TBDMS derivatives hasfound widespread use in organic chemistry but hereto for has not beenused as described herein. The selective desilylation of silyl-protectedsurface functional groups to generate moderately hydrophilic surfacesrepresents a new application of such protection agents, useful in theefficient assembly of functional, siloxane-based multilayeredelectro-optic structures. Moreover, as discussed below, UV-vis, XRR, andSHG measurements clearly show that robust second-order chromophoric thinfilms can be constructed with sub-nm level control of layer dimensionand microstructural acentricity completely preserved as layer-by-layerSA progresses.

The noncentrosymmetric alignment of the high-β chromophores separated bya thin polysiloxane film in each identical layer of the superlatticeresults in very high second-order electro-optic response, competitivewith that of the highest efficiency chromophoric LB films and poledpolymers. Importantly, electric-field poling is unnecessary to establishbulk second-order nonlinearity, suggesting greatly simplified devicefabrication. (Neither electric field poling, poling electrodes, norelectrically matched buffer layers are required). The inventiveprotection-deprotection layer-by-layer SA strategy can be applied toother chromophore components including high-β heterocylic chromophores,such components as would be known to those skilled in the art, andsubsequently toward the integration of such SA materials into devicestructures.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 shows structures of stilbazolium (1A) and related highβ-chromophores (1B-C) of the type which can be silylated and used inconjunction with the methods of this invention.

FIG. 2 shows the reaction of the chromophore precursor 1A with excess of1-iodo-n-propyl-3-trimethoxysilane or Mel in dry THE at 90° C. to give2A and 3A, respectively.

FIG. 3 is a schematic representation of the self-assembly of covalent2-based chromophoric superlattices by deprotection of theTBDMS-derivatized hydroxyl groups with ^(n)Bu₄NF and treatment of theresulting film with octachlorotrisiloxane.

FIGS. 4 a-c show linear dependence with the number of bilayers of threephysical properties of the chromophoric superlattices: (a) Transmissionoptical absorbance (abs; arbitrary units) at λ=580 nm. (b) Thickness (d)in Å derived from specular X-ray reflectivity measurements. (c) Squareroot of the 532 nm SHG intensity (1^(2ω); arbitrary units).

FIG. 5 provides normalized X-ray reflectance plotted versus wave vectorfor a sample with 5 layers.

FIG. 6 plots SHG intensity as a unction of fundamental beam incidentangle from a float glass slide having a SA multilayer (n=5) on eitherside.

FIG. 7 shows a characteristic second harmonic generation (SHG)interference pattern, as demonstrated with a 100 nm-thick C-based film(FIGS. 1 and 2).

FIG. 8 compares, schematically, integration of large responseelectro-optic materials into device structures.

FIG. 9 illustrates electro-optic modulation with a C-based 50 nm thickfilm of this invention (see, also, FIGS. 1 and 2).

FIG. 10 demonstrates the transparency of the prototype electro-opticmodulator of the type illustrated in FIG. 8.

FIG. 11 shows, structurally, other chromophores of the type which can bedevised in accordance with the present invention and used with themethods described herein.

FIG. 12 compares, with reference to FIG. 11, characteristic secondharmonic generation (SHG) interference pattern for E and F-based films.

FIG. 13 illustrates a synthetic procedure enroute to chromophore F ofFIG. 11.

FIG. 14 provides physiochemical data characterizing chromophore F.

FIG. 15 illustrates other chromophore substructures of the type whichcan be modified in accordance with this invention and used as describedherein.

DETAILED DESCRIPTION OF THE INVENTION

The generally applicable SA method of this invention involves theiterative combination of: (i) covalent chemisorption of polar monolayersof high-β chromophores (see FIGS. 1 and 2, for instance), (ii) selectiveremoval of tert-butyldimethylsilyl (TBDMS) protecting groups from thesurface bound chromophore films with tetra-n-butylammonium fluoride(^(n)Bu₄NF) in THF to generate a large density of reactive hydroxylsites, and (iii) reaction of each ‘deprotected’ chromophore layer with aheptane solution of octachlorotrisiloxane (150:1 v/v). This ‘capping’step deposits a thin polysiloxane film (preferrably ˜8 Å thick) andpromotes formation of an acentric multilayer structure since it appearsto provide structural stabilization/planarization via interchromophorecrosslinking. (W. Lin, W. Lin, G. K. Wong, T. J. Marks, J. Am. Chem.Soc. 1996, 118, 8034-8042; W. Lin, S. Yitzchaik, W. Lin, A. Malik, M. K.Durbin, A. G. Richter, G. K. Wong, P. Dutta, T. J. Marks, Angew. Chem.Int. Ed. Engl. 1995 34, 1497-1499.) The present liquid-solid interfacelayer-by-layer construction can be carried out conveniently in a singlereaction vessel, whether by batch or another suitable process. Thethermally and photochemically robust thin film superlattices exhibitvery high second-order responses (χ⁽²⁾ as large as ˜220 pm/V), adherestrongly to the sodium lime glass, silicon, or indium tin oxide-coatedglass (ITO) substrates, and are insoluble in common organic solvents.

With reference to several examples and figures, the present inventionincludes preparation of 50 and 100 nm thick C-based (see FIGS. 1 and 2)films on sodium lime glass, silicon, indium tin oxide-coated glass(ITO), and SiO₂-coated gold, and the integration of these electro-opticmaterials into modulator-type structures. The thermally andphotochemically robust thin film superlattices adhere strongly to thesubstrates, and are insoluble in common organic solvents (e.g.: toluene,methanol, acetone, tetrahydrofuran, pentane or heptane). The structuralregularity of the films has been studied by angle-dependent polarizedsecond harmonic generation (SHG). The polarized angle-dependent secondharmonic generation measurements were made in the transmission modeusing the λ_(o)=1064 nm output wavelength of a Q-switched Nd:YAG laser.The characteristic second harmonic generation interference patternclearly shows that the quality and uniformity of the organic films isidentical on both sides of the float glass substrate. (See FIG. 7)

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the compositions, materials, devices and/ormethods of the present invention, including the assembly of chromophoricand non-linear optic films and materials as are available through thesynthetic methodology described herein. In comparison with the priorart, the present methods and materials/devices provide results and datawhich are surprising, unexpected and contrary to the prior art. Whilethe utility of this invention is illustrated through the use of severalfilms, materials and/or devices and the molecular components thereof, itwill be understood by those skilled in the art that comparable resultsare obtainable with various other compositions, films and/or devices,commensurate with the scope of this invention.

As shown below, the iterative chemisorptive SA process and thestructural regularity of the resulting multilayers have beenunambiguously characterized by a full complement of physicochemicaltechniques: optical transmission spectroscopy, advancing aqueous contactangle (CA) measurements, X-ray photoelectron spectroscopy (XPS), X-rayreflectivity (XRR), and angle-dependent polarized second harmonicgeneration (SHG). The XRR measurements are crucial in establishing filmsub-nanostructural order, thickness, surface roughness, and density.

Toluene, pentane, and heptane were dried over Na/K and distilled undernitrogen. Elemental analyses were carried out at Midwest Microlab, LLC,Indianapolis, USA. The reagent 3-iodo-n-propyltrimethoxysilane waspurchased from Gelest, Inc, PA (USA). Octachlorotrisiloxane was preparedaccording to a literature procedure. (Formation ofoctachlorotrisiloxane: W. C. Schumb, A. J. Stevens, J. Am. Chem. Soc.1947 69, 726-726.) Silicon wafers (Semiconductor Processing Company) andsodium lime glass were cleaned by immersion in a freshly prepared‘piranha’ solution (conc. H₂SO₄:H₂O₂ 30%=7:3 v/v) at 80° C. for at least45 min. This solution is a very strong oxidizing agent and should behandled carefully. After cooling to room temperature, the slides wererinsed repeatedly with de-ionized (DI) water and subjected to anRCA-type cleaning procedure (H₂O:H₂O₂ 30%:NH₄OH 5:1:1 v/v/v, sonicatedat room temperature for at least 45 min). The substrates were thenrinsed with DI water and dried in an oven overnight at 115° C. ITO glasssheets were used for XPS measurements and were purchased from DonnellyCorporation and cut into 2.52×2.54 cm square pieces. The ITO substrateswere sequentially washed with isopropyl alcohol, acetone, and methanolin an ultrasonic bath for at least 30 min, and dried in an ovenovernight at 115° C. Advancing aqueous contact angles (θ_(a)) weremeasured on a standard goniometer bench fitted with a Telfon micrometersyringe at room temperature. All reported values are the average of atleast 5 measurements taken on both sides of the glass substrates.

Spectroscopic Analysis. ¹H NMR spectra were recorded at 300.1 MHz on aVarian Gemini 300 spectrometer. All chemical shifts (δ) are reported inppm and coupling constants (J) are in Hz. The ¹H NMR chemical shifts arerelative to tetramethylsilane (TMS). The resonance of the residualprotons of the deuterated solvent was used as an internal standard (δ2.05 acetone; δ 7.26 chloroform). UV-vis spectra were recorded with aCary 1E spectrophotometer. Polarized angle-depended SHG measurementswere made in the transmission mode using the 1064 nm output of aQ-switched Nd:YAG laser operated at 10 Hz with a pulse width of 3 ns.The details of the setup can be found elsewhere. (S. Yitzchaik, S. B.Roscoe, A. K. Kakker, D. S. Allan, T. J. Marks, Z. Xu, T. Zhang, W. Lin,G. K. Wong, J. Phys. Chem. 1993 97, 6958-6960.) The data werereproducible over a range of points on the sodium lime glass and ITOsamples, and were directly calibrated against those from a Y-cutα-quartz reference. The intensity variation for samples preparedsimultaneously was less than 10%. The X-ray reflectivity measurementswere performed at Beam Line X23B of the National Synchrotron LightSource at Brookhaven National Laboratory in Upton, N.Y., USA. Details ofthe data acquisition and analysis procedure were reported previously.(A. Malik, W. Lin, M. K. Durbin, T. J. Marks, P. Dutta, J. Chem. Phys.1997 107, 645-651.) XPS measurements were carried out using the Al Kαsource of a VG ESCALAB MKII photoelectron spectrometer at the Universityof Arizona in Tucson, Ariz., USA.

Example 1

Synthesis of4-[[4-[N,N-bis((tert-butyldimethylsiloxy)ethyl)amino]phenyl]azo-1-alkyl-pyridiniumiodide salt (alkyl=methyl, n-propyl-3-trimethoxysilane). For Cpd 2 (FIG.2). A dry THF solution (15 mL) of 1 (300 mg, 0.583 mmol) and 6.5 equivof 3-iodo-n-propyltrimethoxysilane (750 μL, 3.81 mmol) was loaded intopressure vessel (25 mL) and heated overnight at 80° C. under Ar. The THFand the excess 1-iodo-n-propyl-3-trimethoxysilane were then removedunder high vacuum at 80° C. affording compound 3 as a dark purple solidin quantitave yield. ¹H NMR analysis of the reaction mixture after 1 hindicated ˜15% formation of 2 and starting materials. No other productswere observed. Anal. Calcd. for C₃₃H₆₁I₁N₄O₅Si₃: Calcd C 49.24; H 7.64.Found: C 49.64; H 7.35. UV-vis (toluene): λ_(max)=536 nm. ¹H NMR(CD₃COCD₃): δ9.21 (d, ³J_(HH)=7.1 Hz, 2H, ArH), 8.25 (d, ³J_(HH)=6.7 Hz,2H, ArH), 8.00 (d, ³J_(HH)=9.0 Hz, 2H, ArH), 7.17 (d, ³J_(HH)=9.5 Hz,2H, ArH), 4.83 (t, ³J_(HH)=7.3 Hz, 2H, N—CH₂), 4.02 (t, J_(HH)=5.0 Hz,4H, OCH₂) 3.94 (t³, J_(HH)=5.0 Hz, 4H, NCH₂), 3.57 (s, 9H, Si(OCH₃)₃),2.11 (m, 2H, CH₂), 0.91 (s, 18H, C(CH₃)₃), 0.78 (t, ³J_(HH)=8.4 Hz, 2H,CH₂Si), 00.7 (s, 6H, SiCH₃). For 3. A THF-d₈ solution (1.5 mL) of 1 (60mg, 0.117 mmol) and Mel (100 μL, 1.61 mmol) was loaded into a 5-mmscrewcap NMR tube and heated overnight at 80° C. The resulting darkpurple solution was analyzed by ¹H NMR, showing unreacted Mel and theselective formation of 3. No other products were observed. Compound 3was obtained quantitatively as a dark purple solid after evaporation ofthe solvent and the excess of Mel. Anal. Calcd. for C₂₈H₄₉I₁N₂O₂Si₂H₂O:Cald: C 49.56; H 7.32. Found: C 49.84; H 7.62. UV-vis (toluene):λ_(max)=536 nm. Mp=158° C. ¹H NMR (CDCl₃): δ9.49 (d, ³J_(HH)=6.8 Hz, 2H,ArH), 8.08 (d, ³J_(HH)=6.9 Hz, 2H, ArH), 7.90 (d, ³J_(HH)=9.3 Hz, 2H,ArH), 7.07 (d, ³J_(HH)=9.4 Hz, 2H, ArH), 4.65 s, 3H, N—CH₃), 3.95 (t,³J_(HH)=5.1 Hz, 4H, OCH₂), 3.85 (t, ³J_(HH)=4.8 Hz, 4H, NCH₂), 0.90 (s,18H, C(CH₃)₃), 0.06 (s. 12H, CH₃).

Example 2

Formation of chromophoric superlattices: (i) Self-assembly of Cpd 2(FIG. 3). Under an Ar atmosphere, the freshly cleaned ITO, sodium limeglass, and/or silicon substrates were loaded into a Teflon sample holderand totally immersed in a dark purple toluene solution of 2 (2.0 mM) forat least 12 h at 90° C. After cooling the Schlenck-type reaction vesselto 25° C., the purple substrates were thoroughly washed with excesstoluene and THF, sonicated in acetone for at least 5 min., and dried atroom temperature under high vacuum. Longer reaction times (up to 40 h)and other organic solvents for rinsing such as hexane, pentane, ormethanol can be used as well. (ii) Deprotection of the TBDMS derivatizedhydroxyl moieties. The substrates were immersed in a freshly preparedTHF solution of ^(n)BuN₄F (0.06 mM) for 4 min, washed with excess THFand MeOH, sonicated in acetone for at least 5 min., and dried at roomtemperature under high vacuum. (iii) Self-assembly ofoctachlorotrisiloxane. Under an Ar atmosphere, the substrates wereimmersed in a dry heptane solution of octachlorotrisiloxane (34 M) for30 min, washed twice with dry pentane, sonicated in acetone for 15 min,and dried at 115° C. for 10 min. The substrates were cooled to 25° C.under high vacuum or by passing a gentle stream of dry Ar through thereaction vessel before repeating step (i). The chromophore 2 andoctachlorotrisiloxane solutions can be used for at least 6 layers.

Example 3

Reaction of chromophore precursor4-[[4-[N,N-bis((tert-butyldimethylsiloxy)-ethyl)amino]phenyl]azo]pyridine1 (180 mM) with a 7-fold excess of 1-iodo-n-propyl-3-trimethoxysilane indry THF at 90° C. results in quantitative formation of the new purple4-[[(4-(N,N-bis((tert-butyldimethylsiloxy)ethyl)amino]-phenyl]azo)-1-n-propyl-3-trimethoxysilane-pyridiniumiodide salt 2 (FIG. 2). The TBDMS functionality is used for protectionof the hydroxyl groups and is introduced withtert-butyldimethylchlorosilane in the presence of imidazole. Compound 2was fully characterized by ¹H NMR and UV-vis spectroscopy, and byelemental analysis. Moreover, the analogous 1-methylpyridinium salt (Cpd3, FIG. 2) can be readily prepared using methyl iodide and has similarspectroscopic properties. For instance, the UV-vis spectra of Cpds 2 and3 in toluene exhibit a characteristic red-shift of the charge-transfer(CT) band to λ_(max)=536 nm in comparison to chromophore precursor 1(Δδ_(max) ˜93 nm).

Example 4

The deprotection of the TBDMS derivatized hydroxyl groups (step ii) wasexamined by specular XRR, XPS, and CA measurements, which unequivocallyreveal the removal of the TBDMS groups. XRR and XPS measurements on aCpd 2-based monolayer reveal an initial film thickness of 14.5±0.5 Å anda Si/N ratio of 0.75, respectively. Treatment of this monolayer with^(n)Bu₄NF (0.06 mM) in THF for 4 min. at 25° C. results in a decreaseof: (i) film thickness by ˜2.6 Å to 11.9±0.5 Å, and (ii) total number ofelectrons per unit area by ˜16%. XPS measurements on the deprotectedmonolayer reveal a Si/N ratio of 0.42. Complete removal of the TBDMSgroup is expected to result in a decrease of the total number ofelectrons per unit area of ˜32% and a Si/N ratio of 0.25. Therefore, ourobservations indicate the loss of ˜50% of the TBDMS protecting groups.Further evidence of hydroxyl functionality deprotection is obtained byCA measurements which show a decrease in θ_(a) of ˜35° to 51±4°. It isknown that in solution, treatment of 2 with ^(n)Bu₄NF at 25° C. resultsin quantitative formation of4-[[4-[N,N-bis(hydroxyethyl)amino]pheny]azo]pyridine by selectivecleavage of both Si—O bonds. The two TBDMS groups of each surface boundchromophore 2 may be chemically inequivalent (i.e., only one TBDMS groupis present near the surface), resulting in a sterically less accessibleTBDMS moiety for nucleophilic F attack. In support of this hypothesis,XRR reveals that the surface roughness (σ_(film-air)) decreases by ˜1.3Å to 3.8±0.5 Å upon treatment of a 1-based monolayer with ^(n)Bu₄NF.Remarkably, the XRR-derived σ_(film-air) width of the ‘deprotected’2-based monolayer is comparable to that of a highly orderedself-assembled octadecyltrichlorosilane film on silicon, demonstratingthat this new ‘protection-deprotection’ SA approach results in formationof smooth, well-organized films.

Example 5

The linear dependence of both the optical HOMO-LUMO CT excitationabsorbance at λ_(max)=580 nm and the XRR-derived film thickness on thenumber of assembled bilayers unambiguously demonstrates that equalpopulations of uniformly orientated chromophores are deposited in eachlayer (FIGS. 4 a,b). From the slope of the XRR measurements, an averageinterlayer spacing of 20.52±10.45 Å can be deduced. A maximum in thereflected intensity at K_(Z)=0.32 Å⁻¹ is observed, which corresponds toa ‘Bragg’ peak due to scattering from essentially identical individualchromophore layers (FIG. 5). The interlayer spacing calculated from thispeak is 19.6±1 Å. Advancing aqueous CA measurements are in accord withexpected surface wettabilities and repeat regularly in each step of thelayer-by-layer SA process: TBDMS (i), ˜86°; ethanolamine (ii), ˜50°;—Si—OH (iii), ˜25°. (W. Lin, W. Lin, G. K. Wong, T. J. Marks, J. Am.Chem. Soc. 1996 118, 8034-8042; W. Lin, S. Yitzchaik, W. Lin, A. Malik,M. K. Durbin, A. G. Richter, G. K. Wong, P. Dutta, T. J. Marks, Angew.Chem. Int. Ed. Engl. 1995 34, 1497-1499.)

Example 6

Angle-dependent SHG measurements are an excellent tool forcharacterization and identification of self-assembled (aminophenyl)azo]pyridinium mono- and multilayer films. Polarized angle-dependent SHGmeasurements on the present films were made in the transmission modeusing the λ_(o)=1064 nm output wavelength of a Q-switched Nd:YAG laser.Details of the experimental set-up have been reported. The intensity ofthe SHG light (1^(2ω)) from a regular multilayered structure shouldscale quadratically with the number of layers (N. Bloembergen, P. S.Pershan, Phys. Rev. 1962 128, 606-622), because the wavelength of theincident light is large compared to the average film thickness(1=20.52(±0.45)×n Å; n=number of layers). The observed linear dependenceof the square root of the second harmonic response on the number oflayers indicates both uniform chromophore alignment and structuralregularity in layer thicknesses (FIG. 4 c), which is fully in agreementwith the UV-vis and XRR data (FIGS. 4 a,b; vide supra). Thecharacteristic SHG interference pattern for each layer clearly showsthat the quality and uniformity of the organic film is identical on bothsides of the float glass substrate and can be fit to eq 1, where ψ isthe average orientation angle between the surface normal and theprincipal molecular tensor component (FIG. 6, n=5). A very large bulksecond-order nonlinear susceptibility response, χ⁽²⁾ zzz ˜5.3×10⁻⁷ esu(˜220 pm/V), and an average chromophore orientation angle, ψ˜36°, isobtained by calibrating the SHG data against quartz. An averagechromophore surface density N_(S) of ˜2×10¹⁴ molecules/cm² is estimatedfor each layer using the ZINDO-derived molecular hyperpolarizability(βzzz) value of 983.70×10⁻³² esu at 1064 nm (W. Lin, W. Lin, G. K. Wong,T. J. Marks, J. Am. Chem. Soc. 1996 118, 8034-8042), the experimental χ²zzz 5.3×10⁻⁷ esu, and the average interlayer spacing 1 of 20.52±0.45 Å(eq 2). The estimated N_(S) value corresponds to an average ‘footprint’of about 50 Å²/chromophore. $\begin{matrix}{\frac{\chi^{{(2)}_{zzz}}}{\chi^{{(2)}_{zyy}}} = {2\cot^{2}\psi}} & (1) \\{N_{B} = \frac{{lx}\quad\chi^{{(2)}_{zzz}}}{\beta_{zzz}x\quad\cos^{3}\psi}} & (2)\end{matrix}$

Example 7

More particularly, a 50 nm thick film can be integrated into a hybriddevice-type structure, based on an all-polymer waveguide structure.(See, FIG. 8). The performance of this self-assembled modulator is beingcharacterized. Preliminarily, modulation has been observed upon applyinga potential across the two gold electrodes (FIG. 9). A controlexperiment with a device lacking the self-assembled superlattice did notexhibit this phenomenon. The switching potential is currently ratherhigh (˜280 V) for practical applications. Modulation at lower potentialscan be achieved in accordance with the present invention by (i)replacing the fluorinated cytop™ layer with a conducting polymer ortransparent conducting oxide (TCO) film, (ii) increasing theself-assembled film thickness, (iii) incorporation of ultra high-βchromophore-based superlattices, and (iv) improving the deviceprocessing procedure. The transparency of the prototype electro-opticmodulator is highly angle-dependent on the polarization of the inputlight (FIG. 10). Maximum transmission is observed at 15° and 130°,respectively.

Example 8

The iterative chemisorptive self-assembly process of this invention andthe structural regularity of the resulting E-based multilayers (FIG. 11)have been unambiguously characterized by a full complement ofphysicochemical techniques: optical transmission spectroscopy, advancingaqueous contact angle measurements, X-ray photoelectron spectroscopy,X-ray reflectivity, and angle-dependent polarized second harmonicgeneration.

F-based monolayers (FIG. 11) have been obtained and identified byoptical transmission spectroscopy, aqueous contact angle measurements,X-ray reflectivity, and angle-dependent polarized second harmonicgeneration measurements. Remarkably, the electro-optic response is about3-4 times than that of a similar E-based film (χ⁽²⁾ is estimated tobe >600 pm/V) (FIG. 12).

Example 9

The following step-wise procedure is provided with respect to thesynthetic sequence illustrated in FIG. 13. All reactions were carriedout under inert atmosphere. Solvents were dried, distilled, and degassedbefore use. The compounds were characterized using conventionalanalytical techniques.

Step 1. An aqueous solution of NaNO₂ (0.45 g, ˜5 mL) was added portionwise to a slurry of 6-amino-2-pyridin-4-yl-benzothiazole (1.42 g, 6.26mmol) in H₂SO₄ (25 mL, 4 M). The mixture became clear and was added to aslurry of N-phenyldiethanolamine (1.1 g, 6.1 mmol) in 50 mL H₂O at 0° C.A dark red solution formed instantaneously and was immediatelyneutralized with aqueous NaOH (9 g, 20 mL). After subsequent stirring atroom temperature for 1½ h a dark red precipitate was obtained byfiltration. TLC (ethyl-actetate) indicated the presence of a new productand N-phenyldiethanolamine. Recrystallization from isopropyl alcoholyielded the pure dihydroxy product (1.15 g, 45%).

Step 2. The dihydroxy compound (0.46 g, 1.1 mmol) was reacted with(tert-butyl)dimethyl-chlorosilane (0.63 g, 4.2 mmol) and imidazole (0.63g, 9.3 mmol) in 5 mL anhydrous THF for 7 h under Ar. TLC (ethyl-acetate)after 1 h indicated that no starting material remained. The crude wasdried under high vacuum overnight at room temperature, dissolved inhexane (100 mL) and washed (4×) with H₂O (5 mL) till neutral. The hexanewas removed by rotavap amd the orange disilyl material was dried underhigh vacuum (0.11 g, 16%).

Step 3. A dry THF solution (7 mL) of the disilyl compound (100 mg, 0.15mmol) and 6.5 equiv of 3-iodo-n-propyl-trimethoxysilane (200 μL, 1.27mmol) were loaded into a pressure vessel (25 mL) and heated overnight at90° C. under Ar. The THF and excess 1-iodo-n-propyl-3-trimethoxy-silanewere then removed under vacuum at 80° C. The resulting solid was washedwith dry pentane. The ammonium product was dried under high vacuum (65mg, 0.07 mmol).

While the principles of this invention have been described in connectionwith specific embodiments, it should be understood clearly that thesedescriptions are added only by way of example and are not intended tolimit, in any way, the scope of this invention. Other advantages andfeatures will become apparent from the claims hereinafter, with thescope of those claims determined by the reasonable equivalents asunderstood by those skilled in the art.

1. A method of using silyl chemistry to control the reactivity of aself-assembled molecular electro-optic material, said method comprising:providing an electro-optic material comprising a silyl-derivatizedchromophore; desilylating said chromophore compound to generate terminalhydroxy functionalities; and reacting said hydroxy functionalities witha reagent having at least one silicon moiety.
 2. The method of claim 1wherein said chromophore is a high-β chromophore.
 3. The method of claim1 wherein said chromophore is derivatized with a trialkylsilylprotecting group.
 4. The method of claim 1 wherein said chromophore isdesilylated by treatment with a deprotecting agent.
 5. The method ofclaim 1 wherein said chromophore is derivatized with atert-butyldimethylsilyl protecting group.
 6. The method of claim 5wherein said chromophore compound is desilylated with a quaternaryammonium fluoride.
 7. A non-linear optical material comprising aplurality of molecular bilayers, each said bilayer comprising a firstchromophore molecular layer coupled to a capping molecular layer with asiloxane bond sequence, said capping molecular layer directly coupled toanother chromophore molecular layer with a siloxane bond sequence. 8.The material of claim 7 wherein said chromophore is a high-βchromophore.
 9. The material of claim 7 wherein said capping layer is apolysiloxane.
 10. The material of claim 9 wherein said capping layercomprises octachlorosiloxane.
 11. The material of claim 7 wherein saidbilayers are deposited on a substrate.
 12. The material of claim 11wherein said substrate and said bilayers are incorporated into awaveguide device.
 13. A chromophore composition with non-linear opticalproperties having the structural formula (Ch)XR_(n), wherein (Ch)Xcomprises a pyridinium chromophore substructure and X is a heteroatom; Ris a trialkylsiloxyalkyl moiety; and n is the number of said moietiesmeeting the valence requirement of said heteroatom.
 14. The compositionof claim 13 wherein said chromophore is selected from the groupconsisting of


15. The composition of claim 13 where in X is selected from the group ofheteroatoms consisting of O and N.
 16. The composition of claim 15wherein X is N and n is
 2. 17. The composition of claim 13 comprising anon-linear optical film.